How to compare datenum variables

Hello,

I have the following problem. I should compare some timestamps I have read from an excel file. Basically what I do is creating a vector containing this timestamps converted using the "datenum" command. If i then for example try to visualize the value of the variable, I get the following result on the console:

datenum(textdata{1})

ans =

7.3450e+005

>> datestr(ans)

ans =

01-Jan-2011 00:00:06

until this point I get it perfectly.

Now, if I have another timestamp which is for example:

01-Jan-2011 00:00:01

and I convert it to a numdate, i get the same result due to the precision which matlab uses to visualize my variable:

datenum(textdata2{1})

ans =

7.3450e+005

now if I wanna compare them, I was guessin, I can just make a simple if test:

a = 01-Jan-2011 00:00:06; b = 01-Jan-2011 00:00:01;

if a == b disp ('equal') elseif a > b disp ('greater') else disp('smaller') end

greater

So up to this point is everything ok.

After that i increase b of the right quantity to make it the same as a:

b = b + (5/(3600*24)); %now b is 01-Jan-2011 00:00:06;

if a == b disp ('equal') elseif a > b disp ('greater') else disp('smaller') end

smaller

What is happening here? why I don´t get that they are the same? For instance this test gives me 'equal' only if I directly assign a = b.

How can I manage this situation?

Answer 1

[Jan]

DATENUM creates a numerical representation of the date with high accuracy, only the display in the command window is abbreviated:

datenum('01-Jan-2011 00:00:01')
>> 7.3450e+005

format long g
datenum('01-Jan-2011 00:00:01')
>> 734504.000011574

See format.

If you convert the date to a string, adding a numerical value is not valid. For a serial date number adding a numerical value is accompanied by rounding, e.g. as in 0.1+0.2~=0.3, see FAQ: Limited precision.

date1 = datenum('01-Jan-2011 00:00:01');
date2 = datenum('01-Jan-2011 00:00:06');
date2 - (date1 + 5/86400)
% >> -1.16415321826935e-010

In consequence arithmetical operations with dates are more accurately performed in datevec format. And a comparison of date strings is done by strcmp.